The present application relates generally to output stages for analog circuits, and more specifically to high output current wideband output stages for amplifiers.
Amplifiers are known that employ a number of stages to produce amplification of electrical signals. A conventional amplifier generally includes one or more amplifying stages followed by an output stage. Output stages typically serve a number of functions. For example, an output stage may be included in a buffer amplifier to provide (1) an appropriate output impedance, (2) a large output voltage swing, and/or (3) a large output current drive. Output stages may also be configured to provide additional amplification for amplifier circuits.
Conventional amplifier output stages have drawbacks, however, particularly when the output stages are implemented using bipolar junction transistors (BJTs). This is because for output stages that include BJTs, high beta (β) and good saturation characteristics are often traded-off against amplifier speed. For example, because the β of high frequency BJTs is normally relatively low, the current gain of such transistors can also be relatively low. As a result, when high frequency BJTs are employed in the output stage of a buffer amplifier, large bias current sources are usually required to enable the output stage to produce a sufficiently large output current drive. However, such large current sources can significantly increase the quiescent current of the output stage, thereby increasing the overall power requirements of the amplifier.
One way of increasing the current gain of an amplifier is to increase the number of stages. However, although the inclusion of additional stages may improve the output drive current capability of an amplifier, such an approach can increase the quiescent current, reduce the output voltage swing, and reduce the bandwidth of the amplifier.
It would therefore be desirable to have an improved output stage for an amplifier. Such an output stage would include high frequency transistors and would provide high output current drive capability without sacrificing amplifier speed. It would also be desirable to have an output stage that maintains a high signal current-to-quiescent current ratio for increased amplifier efficiency.